1. Field of the Invention
The present invention relates generally to fluid velocity sensing apparatus and, more specifically, to such apparatus which include the capability of compensating for variations in fluid temperature over a predetermined operating range. Apparatus of this type find application in many process control situations wherein it is necessary to monitor and sense the velocity or flow rate of a fluid stream, which may be either a liquid or a gas. The invention disclosed herein has been developed as the result of a need for sensing air velocity or flow in order to provide proper control in air conditioning applications.
2. Description of the Prior Art
U.S. Pat. No. 3,623,364 discloses a probe for indicating the flow of fluid which includes a heater element and control circuitry operable to maintain the heater element at a constant, predetermined temperature. A flow sensing temperature measuring device is disposed at an end portion of the probe which is exposed to fluid flow, as well as to the heating effect of the aforementioned heater. The temperature sensed thereby is compared with an "ambient" temperature signal in order to arrive at the appropriate velocity signal. The apparatus disclosed and claimed by applicants differs over the patented system in that the heater means in applicants' second probe means is controlled so as to maintain a constant temperature difference between the heater temperature and the fluid temperature, rather than maintaining a constant heater temperature.
U.S. Pat. No. 3,335,606 also discloses apparatus for sensing the velocity of a fluid stream which includes two thermistors, one of which is disposed within the fluid stream and includes heater means associated therewith, the second of which is exposed only to the temperature of the fluid stream. The thermistors are connected in a wheatstone bridge circuit which, upon becoming imbalanced actuates a heater supply in order to increase the current flowing the aforementioned heater element. The current flowing to the heater element is then measured as an indication of the fluid velocity flowing over the first thermistor.
U.S. Pat. No. 4,028,689 discloses a calorimetric flow monitor which includes first and second thermistors disposed within a fluid flow stream, one of which is heated by a heater element to which a constant current supply is provided. The temperature difference between the respective thermistors is then utilized as an indication of flow rate.
U.S. Pat. No. 3,992,940 discloses a solid-state fluid flow sensor wherein first and second temperature responsive solid-state devices are disposed within a fluid stream, one of which is heated by another solid-state device so as to maintain constant the temperature difference between the aforementioned temperature responsive devices. The current flow to the heater element is monitored as an indicator of fluid velocity.